The present invention relates to a slip control method for controlling the slip of driving wheels of a vehicle, and more particularly to a slip control method for controlling the slip of the driving wheels during starting or acceleration of the vehicle. The instant method of slip control may be employed with systems disclosed in commonly owned U.S. patent applications, Ser. No. 021,892, filed Mar. 4, 1987 and entitled "Traction Control System for Controlling Slip of a Driving Wheel of a Vehicle", and Ser. No. 048,424, filed May 11, 1987 and entitled, "Traction Control System for Controlling Slip of a Driving Wheel of a Vehicle". The disclosures of the above patent applications are hereby incorporated by reference.
Generally, when the traction force of the driving wheels during the acceleration or starting up of the vehicle becomes greater than a friction force between a wheel and the road surface [(a coefficient of friction between the wheel and the road surface) x (load on a driving wheel by the vehicular weight (vehicular load))], the driving wheels slip. A slip rate indicating a degree of such slip is obtained by the following equation (I), where V.sub.w denotes a circumferential speed of the driving wheels, and V denotes a vehicle speed (a circumferential speed of driven wheels). EQU .lambda.=(V.sub.w -V)/V.sub.w (I)
The friction force between the wheel and the road surface (namely, a limit value of the traction force of the driving wheels) varies with the slip rate .lambda., and the friction force in the traveling direction (the longitudinal direction) of the vehicle reaches its maximum value at a predetermined value .lambda..sub.0. While the above friction force between the wheel and the road surface is a friction force in the longitudinal direction of the vehicle, a friction force in the lateral direction (side force) decreases with an increase in the slip rate .lambda..
On the basis of this point, there has been proposed a control method for detecting a slip rate .lambda. which maximizes the longitudinal friction force between the wheel and the road surface in order to maximize the driving efficiency of the vehicle and also suppresses the decrease in the lateral friction force between the wheel and the road surface in order to prevent side slip of the vehicle, and for approximating the slip rate .lambda. to the predetermined value .lambda..sub.0. Specifically, in the conventional method, a lower limit value .lambda..sub.1 and an upper limit value .lambda..sub.2 of the slip rate .lambda. define a predetermined range including the predetermined value .lambda..sub.0, and .lambda..sub.1 and .lambda..sub.2 are set according to the vehicle speed V. The torque of the driving wheels is controlled by a driving wheel torque control device according to a value of the slip rate .lambda. calculated from the driving wheel speed V.sub.w and the vehicle speed V. Consequently the circumferential speed V.sub.w of the driving wheels is controlled and the slip rate .lambda. of the driving wheels is feed-back controlled within the predetermined range .lambda..sub.1 -.lambda..sub.2.
In the conventional method as described above, a premise for slip control of the driving wheels is that the driven wheels do not slip, and thus the vehicle speed can be precisely detected by the speed of the driven wheels. In a front wheel drive car the front wheels are the driving wheels and the rear wheels are the driven wheels. Conversely, in a rear wheel drive car, the rear wheels are the driving wheels and the front wheels are the driven wheels. Accordingly, when the brake is applied to the driven wheels, a brake switch or a clutch switch detects such a braking condition to inhibit the slip control of the driving wheels (This method is disclosed in Japanese Pat. Laid-Open No. 60-121129, for example).
In specialized sports motoring rather than normal street driving, a so-called spin turn or hand brake turn is sometimes executed by braking and locking only the rear wheels by means of the parking brake in order to induce slipping of the rear wheels and thus rapidly slide the rear end of the car around. In this case, since the clutch is not disengaged in a front-wheel drive vehicle, the slip control is still operative. Although it is assumed that the vehicle speed is zero by locking the rear wheels (the driven wheels) to stop the rotation thereof, the front wheels (the driving wheels) continue to rotate. Therefore, if the slip rate .lambda. is calculated in accordance with equation (I), it is inaccurately assumed that the driving wheels are slipping. As a result, a fuel cut control or the like is generated in order to reduce the driving force. When the vehicle is then accelerated just after such a hand brake turn, acceleration performance is reduced, or engine stall occurs upon the hand brake turn due to the operation of the slip control system. Therefore, in the above conventional system, the undesirable result of reduced acceleration performance or engine stall may occur as the result of a hand brake turn.
Additionally, according to the conventional slip control method described above, even when an accelerator pedal is rapidly depressed when starting movement of the vehicle, the torque of the driving wheels can be properly controlled to prevent the slip of the driving wheels.
However, in specialized sports motoring rather than normal street driving, slipping the driving wheels is desireable and therefore is intentionally utilized in some cases. For example, when starting a front-wheel drive vehicle on a slippery road surface such as a snowy road or a frozen road, the transmission is shifted to the reverse gear position, and the steering wheel is rotated just enough to provide a small steering angle for the front wheels. Then, under this condition, the accelerator pedal is fully depressed to intentionally slip the front wheels (the driving wheels) and turn the vehicle about the rear wheels in such a manner as to draw a sectoral track. This maneuver is referred to as a back turn. In a vehicle using the conventional slip control method, the driving wheels cannot be intentionally slipped in order to perform a back turn.